JavaScript ES6: Test for arrow function, built-in function, regular function?

Question

Is there an elegant way to tell Harmony\'s slim arrow functions apart from regular functions and built-in functions?

The Harmony wiki states that:

Answer 1

Ron S's solution works great but can detect false positives:

/** Check if function is Arrow Function */
const isArrowFn = (fn) => (typeof fn === 'function') && /^[^{]+?=>/.test(fn.toString());

/* False positive */
const fn = function (callback = () => null) { return 'foo' }

console.log(
  isArrowFn(fn)  // true
)